9. 1 Transport in the xylem of plants

>>> Transpiration is the inevitable consequence of gas exchange in the leaf

Plant leaves are where the process of photosynthesis occurs. It involves the assimilation of carbohydrates(glucose-C6H12O6) using light energy. Carbon dioxide is used as a raw material and oxygen is a waste product. Gas exchange is

essential for photosynthesis to occur. Absorption of CO2 is essential for

photosynthesis, as the waxy cuticle has low permeability, it needs pores on its surface known as stomata.

A common problem among plants and other organisms is having gas exchange without water loss. This loss of water vapor from the leaves and the stem of the plant is known as transpiration.

Water loss is minimized using guard cells. They’re found in pairs on either side of the stoma. The guard cells control the

opening of the stoma and can adjust it. A group of plants without any stomata are known as liverworts.

>>> Plants transport water from the roots to the leaves to replace losses from transpiration

The water leaving through stomata by transpiration is replaced by the water from the xylem.

The water in the xylem climbs the stem through the pull of transpiration combined with the forces of adhesion and cohesion.

Water moves from the soil into the roots by osmosis due to the active transport of minerals into the root.

Once the water is in the root it travels to the xylem through cell walls(apoplast pathway) and through the

cytoplasm(symplast pathway)

>>> The cohesive property of water and the structure of the xylem cells allow transport under tension

The plants are able to transport water very efficiently due to the structure of the xylem vessel.

Water molecules are cohesive, they are closely stuck together. They’re not broken down by negative pressure or suctions. This is caused by the hydrogen bonding.

Properties/Structure of the xylem vessel

The xylem walls are thickened. The reason they can withstand low pressure without collapsing is that the thick walls are saturated with a polymer known lignin.

Xylem vessels are formed from files of cells, lined end to end. Sometimes in flowering plants the cell wall material in areas between adjoining cells is largely removed. The cells’ contents and plasma membrane breakdown.

Mature xylem cells are non-living, so the flow of water along is a passive process. The atmospheric pressure is higher than the pressure inside the xylem cell, but the tube doesn’t collapse due to the rigid

structure.

>>> The adhesive property of water and evaporation generate tension forces in leaf cell wall

Pulling forces(tension) causes the water to move up to the leaves. These forces are generated by leaves and are due to the adhesive property of

water. Water strongly adheres to the cellulose in plant cells

When water evaporates from mesophyll cell walls in the leaf, more water is drawn through narrow cellulose-lined pores in leaf cell walls from the nearest xylem vessels to replace it, generating the tension.

>>> Active uptake of mineral ions in the roots causes absorption of water by osmosis

>> Adaptation of plants in deserts and in saline soils for water conservation Desert

- Succulence - Succulent plants are water hoarders. They store water in stems, root or fleshy leaves; special structures good at moisture retention.- Toleration- Evasion

Saline- Salt excretion - These plants have leaves with glands that excrete salt.- Leave dropping - The plants store the salt in some leaves and if the load becomes too high, they drop the leaves.- Stomata control - The plants can restrict the stomata opening allowing them to conserve fresh water. - Turned leaves - The plants usually turn the leaf to reduce the surface area exposed to the hot sun and to capture evaporating water and increase humidity.